1. Field of the Invention
The present invention relates to a circuit arrangement for supplying voltage and controlling the operating behavior of light-emitting diodes for illumination purposes and a method therefor.
2. Description of the Related Art
The use of light-emitting diodes in indicating devices has already been known for a long time, but the first light-emitting diodes had only a low light output and their application was therefore limited to this region. It is only recently that light-emitting diodes can be produced that have now adequate luminosity to justify use for illumination purposes. As a rule, a multiplicity of light-emitting diodes are combined in this connection in a matrix arrangement (array) in order to form a radiator, such as, is described, for example, in U.S. Pat. No. 6,016,038.
In that case, the light-emitting diodes are usually driven by a constant-current source in which the current flowing through the diode or diodes is determined and regulated to a specified set point value. Said set point value is preferably chosen in such a way that the light-emitting diodes are operated with as high an efficiency as possible. Such a constant-current source comprises a bipolar transistor whose collector is connected to the light-emitting diodes. The emitter of the transistor is connected to ground by means of an ohmic resistor and the control terminal (base) of the transistor is fed back to regulate the current. The diode current is determined by means of the ohmic resistance and is regulated to the desired value by means of a change in the base voltage of the transistor.
In this connection, there is the option of dimming the light-emitting diode by pulse-width modulation of the signal applied to the base terminal of the transistor. The advantage of this method is that the light-emitting diode is either fully driven or not driven at all, which increases the efficiency of the entire circuit. The frequency of the PWM signal is in this case so high that no flickering can be detected by the human eye.
Since the light-emitting diodes and the associated drive circuits (in particular the constant-current source(s)) have to be very tightly packed for use for illumination purposes in order to achieve an adequate luminosity, there is normally the problem of a very high heat evolution in the case of such arrangements. In particular, the bipolar transistor is exposed to a high thermal loading, which, on the one hand, reduces the efficiency of the entire circuit and, on the other hand, requires certain precautions to be taken in order to avoid a failure of the circuit due to an excessive heat evolution. The measuring shunt used to determine the current flowing through the light-emitting diodes produces, in addition, power loss.
It is the object of the present invention to provide a circuit arrangement for supplying voltage and for controlling the operating behavior of light-emitting diodes for illumination purposes in which the abovementioned problems are avoided and a brightness control is made possible.
The object is achieved by a circuit arrangement or by a method according to the invention wherein at least one light-emitting diode is disposed within a load circuit that comprises a resonance element and that is connected to the output of an inverter whose output frequency can be varied and that is in turn connected on the input side to a direct-voltage source. At the same time, the inverter has at least two controllable power switches whose switching frequency can be altered to control the brightness of the at least one light-emitting diode.
The altering of said switching frequency has the result that, because of the resonance element disposed in the load circuit, the current flowing through the light-emitting diode and, consequently, the brightness of the diode is altered. This corresponds substantially to the known method of driving and operating gas-discharge lamps by means of electronic ballasts. This immediately yields the option of using the topology of known ballasts, including already known and used illumination systems, to make possible the central control of a multiplicity of distributively disposed illumination means.
A further advantage results from the fact that, with suitable adjustment of the load circuit, the supply direct voltage delivered by the direct-voltage source can be chosen from a wide range. In this connection, all known circuits suitable for this purpose, for example AC/DC converters, DC/DC converters, step-up or step-down converters, are conceivable as direct-voltage source. As a result of the utilization of the resonance of the load circuit, there is furthermore also the option of working with low supply voltages and, nevertheless, operating series connections of a plurality of light-emitting diodes whose total forward voltage is above the supply voltage. With a correspondingly high supply voltage, very many light-emitting diodes can accordingly be connected in series.
A further advantage of the circuit arrangement according to the invention over the constant-current sources used hitherto is that, if suitable power switches are chosen, only very low switching losses and conducting-state power losses occur in the inverter, with the result that higher efficiency is achieved in total. For example, field-effect transistors can be used as power switches, in which case a further advantage emerges that, compared with the bipolar transistor of a constant-current source, only a very low heat evolution is to be feared in the case of the field-effect transistors of the circuit arrangement of the present invention. The power switches of the inverter may, for example, be disposed in the form of a half-bridge circuit or a full-bridge circuit.
More specific aspects of the invention, which relate to developments described and claimed herein. In order, for example, to utilize both half-waves of the alternating voltage generated by the inverter, it is advantageous to connect a plurality of light-emitting diodes or light-emitting diode arrays in anti-parallel in the load circuit so that they are operated in a pulse mode with a maximum of 50% switch-on time in each case. Another option may be to insert a rectifier directly upstream of the light-emitting diodes or the arrays, which results in 100% switch-on time. In this case, the light-emitting diodes are connected in parallel.
A further development of the circuit arrangement according to the invention is that means are provided to determine the current flowing through the light-emitting diode or light-emitting diodes. This results in the option of regulating the switching frequency of the power switches as a function of the current determined and, thereby, to adjust the entire circuit arrangement to a varying number of light-emitting diodes connected in series without increasing the power loss by doing so. Another option is to regulate the supply direct voltage delivered by the direct-voltage source as a function of the current determined. Furthermore, provision can be made to determine the intensity of the light delivered by the light-emitting diodes and thereby adjust the switching frequency of the power switches to a value that corresponds to a desired luminosity of the entire arrangement.
It would furthermore also be conceivable to use light-emitting diodes of various colors in order to establish overall a desired mixed color by a suitable control of the intensity of the various colors. In this case, an inverter is provided for each color of the light-emitting diodes so that the intensity of the various colors can be controlled independently of one another
An advantageous development of the invention relates to measures which enable the dimensions of the circuit arrangement to be kept as compact as possible. In order to achieve this, the circuit arrangement comprises, at least partly, a multilayer circuit into which passive components (for example capacitors, inductors and the like) are integrated. This integration is possible, in particular, if the power switches are operated at high frequencies since correspondingly lower capacitance values or inductance values can then be used in the circuit. In the present case, a frequency range of 200 kHz to 1 MHz has proved to be particularly suitable. An increased radiation of electromagnetic high-frequency fields first of all due to the increase in frequency can be avoided by suitable screening measures that can easily be undertaken because of the reduced dimensions of the circuit.
Components may be integrated, for example, by means of multilayer printed-circuit-board technology. Preferably, the multilayer circuit is implemented by an LTCC (low temperature co-fired ceramic) structure that comprises a plurality of low-sintering ceramic layers or sheets that are disposed above one another and between which conductor tracks are situated. Compared with conventional printed-circuit-board technology, said LTCC technology, which has newly been developed in recent years and disclosed, for example, in EP 0 581 206 A2, can achieve yet another miniaturization of the circuit. In this technology, inductances and capacitances, in particular, can be integrated into the multilayer circuit in addition to the conductor tracks. Furthermore, the ceramic material offers the advantage that it conducts heat relatively well, which means that, for the same overall volume, greater powers can be achieved since heat loss is radiated better. Preferably, the heat dissipation is increased yet again by encapsulating the ceramic structure in a metallic housing. An efficient screening of the high-frequency fields radiated by the circuit arrangement into the environment can also be achieved in this way.
At the abovementioned frequencies, many of the components of the circuit arrangement can be integrated into the multilayer circuit. The remaining passive components and also semiconductor chips have, however, still to be mounted on the surface or outside the ceramic structure. In order to achieve as small a space requirement as possible for this purpose also, the semiconductor chips are preferably mounted on the ceramic substrate by means of the known flip-chip (FC) technology. In this connection, a plastic layer that is, on the one hand, electrically conductive perpendicular to the connection level and is insulating in the connection level and that, on the other hand, absorbs stresses occurring in the case of a different thermal expansion of the semiconductor chip and of the ceramic substrate and, consequently, prevents destruction of the semiconductor chip is introduced between the semiconductor mounted without a housing and the contacts on the surface of the carrier substrate.